It is possible to artificially constitute a medium that has properties not found in nature by arranging small pieces (unit cells) of a metal, a dielectric, magnetic material and a superconductor at sufficiently shorter intervals than a wavelength (about 1/10 of the wavelength or less). A medium like this is called a metamaterial, in the sense that it is a medium that belongs to a category that is larger than the category of a medium found in the natural world. The properties of a metamaterial change in various ways depending on the shape and material properties of the unit cell, and the arrangement thereof.
Among these, a metamaterial that has an equivalent permittivity ∈ and magnetic permeability μ and is simultaneously negative has been called a “left-handed medium (LHM: Left-Handed Materials)” because its electric field, magnetic field and wave vector form a left-handed system. In contrast to this, a normal medium that has an equivalent permittivity ∈ and magnetic permeability μ and is simultaneously positive is called a “right-handed medium (RHM: Right-Handed Materials)”. The relationship between the permittivity ∈ and magnetic permeability μ and the type of medium is shown in FIG. 1. Mediums can be classified into a first quadrant to a fourth quadrant in accordance with a positive and negative permittivity ∈ and a positive and negative magnetic permeability μ. A right-handed medium is a first quadrant medium, and a left-handed medium is a third quadrant medium.
In particular, the left-handed medium has peculiar properties, such as the existence of a wave (called a backward-wave) having a wave group velocity (propagation velocity of energy) and a phase velocity (progression velocity of phase) with opposite signs, and an amplification of an evanescent wave, which exponentially decays in a nonpropagation region. Now then, it is possible to use a left-handed medium to artificially constitute a transmission line that transmits a backward-wave. This fact is disclosed in Non-Patent Document 1 and Non-Patent Document 2 cited hereinbelow.
Based on this concept of a left-handed medium constitution, a transmission line that propagates a backward-wave by periodically lining up unit cells comprising a metal pattern, is proposed. This transmission characteristic has been handled theoretically until now, and it has become theoretically evident that this transmission line has a left-handed transmission band, that a bandgap occurs between the left-handed transmission band and the right-handed transmission band, and that the bandgap width thereof can be controlled by the reactance among the unit cells. Further, a transmission line that is capable of simultaneously transmitting a left-handed transmission band and a right-handed transmission band is called a composite right/left-handed transmission line. These points are disclosed in Non-Patent Document 3 cited hereinbelow.
FIG. 2 is a diagram showing the constitution of a microstrip line commonly used in the past. FIG. 2(A) is an oblique view of the microstrip line, and FIG. 2(B) is a cross-sectional view showing an overview of the electromagnetic field of an electromagnetic wave propagated over the microstrip line. The microstrip line comprises a conductor 4 that serves as a transmission line disposed on the front surface of a substrate 1 of thickness d comprising a dielectric substance, and a ground conductor 3 disposed on the rear surface of the substrate 1. The electric field E and magnetic field H of the electromagnetic wave propagated over this microstrip line are as shown in FIG. 2(B). Since a half-space is opened on the one side of the transmission line (the top surface side) of the microstrip line, radiation toward the space occurs in the radiation region (region in which the phase constant of the propagation wave of the transmission line is smaller than the in-vacuum wave number).
A right/left-handed transmission line based on the constitution of the transmission line of this type of microstrip line has already been produced, and the transmission characteristics of this microstrip line-type right/left-handed transmission line have been verified experimentally. This is disclosed in Non-Patent Documents 2 and 3. The microstrip line-type right/left-handed transmission line is such that unit cells comprising conductors that are insulated from one another are periodically arrayed in the z direction as conductors 4 in FIG. 2(A).
Because the microstrip line-type right/left-handed transmission line has a property that radiates a portion of the transmission energy in a frequency region in which the phase constant of a wave is smaller than the in-vacuum wave number, it is confirmed that the right/left-handed transmission line can be used as an antenna by taking advantage of this property. This is disclosed in Non-Patent Documents 2 and 3.
In addition to the microstrip line, a stripline has also been used as a transmission line for some time now. FIG. 3 is a diagram showing the constitution of a stripline. FIG. 3(A) is an oblique view of the stripline, and FIG. 3(B) is a cross-sectional view showing an overview of the electromagnetic field of an electromagnetic wave propagated over the stripline. The stripline is such that ground conductors 2 and 3 are disposed on the front surface and rear surface of a substrate 1 of thickness s comprising a dielectric substance, and a conductor 4 is disposed as a transmission line in an intermediate plane of the substrate 1 (plane located at a thickness of s/2). The electrical field E and magnetic field H of the electromagnetic wave propagated over this stripline are as shown in FIG. 3(B). Radiation essentially does not occur because both the front surface and rear surface of the stripline are covered by ground conductors 2, 3.
The inventors have already proposed a composite right/left-handed transmission line and left-handed transmission line based on this stripline-type transmission line constitution. This is the transmission line shown in FIGS. 4(A) and (B). FIG. 4(A) is an oblique view displaying only the conductor that constitutes the transmission line, and FIG. 4(B) is a cross-sectional view of the transmission line. This transmission line is such that ground conductors 2 and 3 are disposed on the front surface and rear surface of a substrate 1 of thickness s comprising a dielectric substance, and a conductor pattern 4 is disposed as a transmission line in an intermediate plane of the substrate 1 (plane located at a thickness of s/2). The conductor pattern 4 is such that unit cells comprising conductors that are insulated from one another are arrayed periodically in the direction of transmission.
[Non-Patent Document 1] D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, “Composite medium with simultaneously negative permeability and permittivity”, Phys. Rev. Lett., vol. 84, no. 18, pp. 4184-4187, May 2000
[Non-Patent Document 2] C. Caloz and T. Itoh, “Application of the transmission line theory of left-handed (LH) materials to the realization of a microstrip LH line”, IEEE-APS Int'l Symp. Digest, vol. 2, pp. 412-415, June 2002
[Non-Patent Document 3] Atsushi Sanada, Christophe Caloz and Tatsuo Itoh, “Characteristics of the Composite Right/Left-Handed Transmission Lines”, IEEE Microwave and Wireless Component Letters, Vol. 14, No. 2, pp. 68-70, February 2004
When a conventional microstrip line-type right/left handed transmission line is used as a leaky-wave antenna, the direction of a radiating electromagnetic wave can be changed by changing the frequency of the electromagnetic wave to be propagated. However, when the frequency variability range is small, the change in the direction of the radiating electromagnetic wave cannot be controlled across a broad range. Further, it is the same when a constitution for radiating an electromagnetic wave is added to a stripline-type right/left-handed transmission line, and this stripline-type right/left-handed transmission line is used as a leaky-wave antenna, and the change in the direction of the radiating electromagnetic wave cannot be controlled across a broad range when the frequency variability range is small.